A dielectric material is a material which, when in the presence of electric field, stores more energy than is stored in free space. That is, the dielectric material stores electrical potential energy inside itself. Many common materials, such as glass and epoxies, act as dielectrics.
This ability of the dielectric is commonly expressed in terms of permittivity. The permittivity for free space is identified as .epsilon..sub.o while relative permittivity for a material, unitless, is identified as .epsilon..sub.r and is equal to the ratio of permittivity of the material to the permittivity of free space. The relative permittivity for air is approximately equal to 1. For purposes of this application, permittivity means relative permittivity unless otherwise indicated.
In the presence of time varying electric fields a dielectric dissipates a portion of its stored energy as heat. This loss is indicated by assigning the permittivity real and imaginary parts for the special case of steady state sinusoidally varying fields.
Thus the complex permittivity is defined by the following equation: ##EQU1##
These real and imaginary parts are in general functions of the frequency (.omega.) so that the permittivity is better expressed as the following: ##EQU2##
See FIG. 1 for a plot of .epsilon.(.omega.) as a function of frequency for a simple material. It is seen that as frequency increases, .epsilon.' attains the value of 1, that is the permittivity of free space, while .epsilon." has one or more peak values at special frequencies. The imaginary part of the permittivity, .epsilon.", acts like the first derivative of .epsilon.'. That is, where the rate of change of .epsilon.' is the greatest, .epsilon." peaks to provide what is called an absorption band.
One of the uses for dielectric materials is in the form of lenses to increase the amount of power that an antenna can collect. Such lenses work because electromagnetic radiation travels slower in the lens than in free space and thus bends. A lens, to be effective, must have a diameter greater than two times the wave length (.lambda.) of the electromagnetic radiation. Since the wave length equals the speed of light divided by the frequency, at a frequency of, for example 200 MHz, the wave length will be about 5 feet. Therefore, extremely large lenses are required at radio frequencies. This makes the use of natural dielectrics, such as glass and epoxies, impractical for radio frequency lens applications because of their weight. To overcome this obstacle investigators in the late 1940's and 1950's demonstrated that artificial dielectric materials could be made.
One type, termed metallic delay media, can be made of sheets of foam material with flat metal squares fixed to at least one of the surfaces, the metal squares being separated from one another. See S. B. Cohn, "The Electric and Magnetic Constants of Metallic Delay Media Containing Obstacles of Arbitrary Shape and Thickness", J. of App. Phys. Vol. 22, No. 5, May 1951 pp. 628-634, the disclosure of which is incorporated by reference. The pieces of foam with the metal squares are then stacked and secured together to obtain a light-weight dielectric. The material can be shaped to create a lens to cause concentration of an electromagnetic wave passed through the material.
These materials behave in a manner similar to a resistor capacitor inductor (RCL) circuit. Thus, by minimizing the resistance of the obstacles a very low loss dielectric can be made. Near the frequency where the obstacles are one half wavelength long, the material behaves as a resonant LC circuit causing the permittivity to be strongly frequency dependent. When this effect is undesirable, the obstacles are made very small, effectively pushing the material resonance outside the desired band of operation. On the other hand, if a designer makes a prism having a dielectric constant which changes rapidly with frequency (and thus with wavelength), a single transmission can be separated in space into its individual frequencies. That is, the signal can be multiplexed. In communication this is quite useful since the more frequencies available, the more information can be transmitted.
Because the early applications of artificial dielectrics were for the purpose of enhancing the transmission of energy, the greatest concern was in obtaining a controlled real part of the permittivity .epsilon.'. The control of .epsilon." came with the need to absorb or attenuate electromagnetic waves.
In its simplest form such a material starts with a host material into which a "lossy" material, like carbon, is introduced and dispersed throughout the volume. This type of dielectric material is termed an absorber.
The permittivity of this type of material is in general frequency dependent and unpredictably so. Even very fine particles of carbon tend to make tiny chains or groups of carbon particles within the host material. The tiny chains are separated by gaps creating a multitude of RC circuit analogs. Because of this tendency, the final complex permittivity is influenced by the amount of carbon present and is sensitive to the process used in making the material. The carbon can align, segregate or clump at random. These random effects reduce the repeatability of these materials and frequently lead to unplanned anisotropic properties. (Materials with anisotropic properties exhibit different dielectric properties depending upon the direction of the electric field.) Therefore, the making of an isotropic (essentially equal dielectric properties in all directions) material using microscopically sized carbon particles by randomly scattering it throughout the volume is undesirable from a manufacturing reliability standpoint.
A different approach to electromagnetic wave absorption is embodied in circuit analog absorbers. FIG. 2 illustrates a single sheet of plastic having a number of cross shaped elements against which a wave is projected. The sheet acts in the manner shown in FIG. 2A; that is, as an RCL series circuit where the resistance is determined by the conductivity of the elements, the capacitance is determined by the spacing and size of the elements, and the inductance depends on the size, width and shape of the elements.
In the embodiment of FIG. 2, part of the wave is reflected, part passes through and part is dissipated within the material. Using this principle, a circuit analog absorber can be designed by stacking a precise number of sheets with specific properties at precise spacings as show in FIG. 2B. In this case the distance d between the sheets is equal to one half the wave length. With this arrangement, the multiple reflections between the wave and the various sheets are precisely balanced with the impedance of each sheet to obtain absorption over a broad band of frequencies.